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Frozen IVF embryos linked to ectopic pregnancy

By Shaoni Bhattacharya, San Antonio

The rate of ectopic pregnancy is over 17 times greater when using frozen embryos for in vitro fertilisation rather than fresh ones, suggests a new US study. Ectopic pregnancies cannot result in the birth of a baby and can endanger the life of the mother.

The team at Brown University in East Providence, Rhode Island, looked at pregnancy rates from over 2800 IVF cycles and found that nearly a third of pregnancies resulting from frozen embryo transfer were ectopic.

In contrast, just two per cent of pregnancies resulting from the implantation of a fresh IVF embryo resulted in an ectopic pregnancy. This rate is similar to that seen with naturally conceived pregnancies.

David Keefe, one of the researchers, says they were surprised by their findings. But he adds&colon; “Obviously more work needs to be done to confirm this.”

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“The burden is now on us and others to figure out why,” he says. He speculates that the actual process of freezing and thawing an embryo could disrupt its development or structure.

But Alison Cook, of the UK’s Human Fertilisation and Embryology Authority, says the study shows unusually high rates. “It’s surprising, I’ve never seen anything like it before. I don’t know how his protocol differs from that used in the UK.”

Thawing process

Cook told New Scientist that frozen embryo transfer is an increasingly common method employed for IVF. This is because it means women only have to undergo one cycle of ovarian stimulation – “which has its risks” – to produce eggs. After fertilization, the resulting batch of embryos can be frozen and used as needed.

The researchers examined pregnancies from 2452 “fresh” IVF cycles and 392 frozen embryo transfer cycles. They found that six of the 19 frozen embryo transfer pregnancies were ectopic compared with just nine of the 490 pregnancies achieved through fresh IVF.

“The reason for this difference remains unknown, however, the developmental delay encompassed by the thawing process may perhaps afford greater opportunity for the embryo to migrate into the fallopian tube prior to hatching,” says the team, which presented the findings at the American Society for Reproductive Medicine’s meeting in San Antonio, Texas, US.

Keefe speculates that the freeze-thawing process might somehow alter the outer layer of cells enclosing the embryo called the zona pellucida. This might cause it to get lodged where it normally would not rest.